1. Field of the Invention
The present invention relates to a synchronizing method in a mobile radio system which lacks a global time reference, i.e. a system which lacks a time reference that is common to all mobiles and base stations in the system. More specifically, although not exclusively, the present invention relates to a method of synchronizing a random bit flow which is superimposed cryptically on the normal traffic flow during call transmission or data transmission between the base stations and the mobiles. The present invention can be applied solely to a so-called TDMA-system, i.e. a mobile radio system in which calls are transferred in frames and time slots by means of time multiplexing.
2. Description of Prior Art
It is desirable in mobile radio systems, and in TDMA-systems in particular, to be able to maintain the security of calls so as to prevent unauthorized persons from setting-up and making calls over the network. In order to meet this requirement, it has been suggested that the calls are ciphered; see for instance EP-A-273289.
Calls between a base station and a mobile are ciphered by processing the speech message in a scrambler, which functions to convert the speech signals into a random sequence in accordance with a given key. For instance, the speech signals can be superimposed with a random sequence of bits of relatively long duration (several minutes). In this case, the cipher key consists in the knowledge of the total bit pattern of the sequence and also knowledge of the time at which the sequence was commenced. An authorized subscriber can plug into a circuit module in the apparatus which stores this sequence and said starting timepoint, and is thus able to decode incoming calls.
Some TDMA mobile radio systems lack a global time reference, i.e. means in the mobile telephone exchange which contains a clock common to the overall system and operative to produce the same time reference for all base stations and mobiles in the system. The purpose of such a common time reference is to enable the mobiles and the base stations to be synchronized with one another in certain frames and time slots when synchronizing from signalling or data/speech signals falls away for some reason or other. When transferring (hand-off) a call from one radio channel to another radio channel, synchronization between the mobile and its base station may be los, since a brief interruption will occur during the actual transmission or switching of the call. If the call is also ciphered, a further problem can occur as a result of fall-off of the synchronism of the actual ciphering key concerned, thereby rendering deciphering impossible.
These problems also occur when setting up ciphered calls, although said problems are most pronounced during "hand-off".